The present invention relates generally to electrical power switches, and particularly to separable connector switch modules for shielded electrical power distribution cables which include ablative material for loadbreak operation.
Separable connector assemblies for underground power distribution cable, or shielded cable, are watertight when assembled and may be readily separated into two or more units to break a cable connection. As such units are available separately commercially and are individually subject to special design considerations, they are commonly referred to as "modules". Thus, a connector assembly includes two or more matching modules assembled together.
One type of separable connector assembly commonly used is a "rod and bore" switching type. A switch module having a receiving bore tube, or snuffer, situated within a passageway in a shielded, insulating housing and a grasping contact member at the interior end of the bore tube receives a matching rod connector module having a rod contact which is inserted in the bore and grasped by the bore contact member. Examples of this type of connector assembly are described, for example, in the following U.S. Pat. Nos.:
3,513,437 issued May 19, 1970 to W. A. Morris
3,542,986 issued Nov. 24, 1970 to E. J. Kotski
3,551,587 issued Dec. 29, 1970 to R. F. Propst, and
3,587,035 issued June 22, 1970 to E. J. Kotski
It is desirable to be able to operate such connectors and interrupt the power while the cables are energized. As the cables are generally carrying power at a voltage on the order of thousands of volts, separation of the contacts of the connector on an energized cable results in the formation of an electric arc between the contacts. The arc will, unless promptly extinguished, eventually strike a ground plane such as the grounded shielding of the modules, and create a direct line-to-ground fault.
In present connectors, the bore of a snuffer tube is lined with ablative material, a material which generates arc-extinguishing gases when subjected to an electric arc. An arc follower, a rod-shaped extension at the end of the metal contact rod is also of ablative material. When the contact rod is pulled from the grasping contacts of the bore module, the resulting arcing passes between the follower and the snuffer liner. The exposure of the ablative material to the arcing causes it to generate arc-extinguishing gases which rapidly extinguish the arc. This permits the connector to be utilized as a switch by being operated under live conditions, without creating a line-to-ground fault.
There is a problem in finding a suitable ablative material for the follower. The material must be sufficiently effective in generating arc-quenching gases while also being strong enough mechanically to withstand a relatively high bending moment which might be inadvertently applied by an operator in joining or separating a matching pair of modules.
It has been found, for instance, that in general the ablative materials most effective for generating arc-quenching gases, such as melamine for example, do not have the necessary mechanical strength and are likely to fracture on operation where the cables to the connector modules are improperly trained.
Various materials with high mechanical strength and which can be loaded with an ablative material such as melamine are not as effective in generating gases of the proper dielectric strength and may have other disadvantages either in the electrical characteristics or in the manufacturing process.
Nylon has a tendency to absorb moisture and to track along the surface under high-voltage conditions until a conductive path is formed. This can lead to an electrical failure. It also requires a relatively high degree of loading with ablative material.
Acrylic resins have less desirable mechanical characteristics than does nylon and is more difficult to mold in the manufacturing process.
Epoxy resins do not have sufficient mechanical strength when loaded with melamine and involve costly production procedures.